1. Field of the Invention
The present invention relates to a recording element substrate configured to perform recording using thermal energy, a method of manufacturing the recording element substrate, and a liquid ejection head.
2. Description of the Related Art
As thermal recording devices in which recording is performed by heating a plurality of heating portions provided therein, sublimation-type recording devices and inkjet-type recording devices are known. In a sublimation-type recording device, recording is performed by a method in which ink on an ink ribbon is melted by heat and transferred onto a recording medium, while in an inkjet-type recording device, recording is performed by a method in which a liquid, such as ink, is subjected to film boiling, thereby ejecting the ink. In these recording devices, in order to diffuse excess heat generated in heating portions, a substrate having high thermal conductivity is used as a recording element substrate (hereinafter, may be also referred to as a “head substrate”). In order to perform continuous recording efficiently, it is necessary to dispose a heat storage layer which stores a certain amount of heat between the heating portions and the substrate. An insulating layer used as an electrode insulation film also serves as the heat storage layer.
However, in the case where a multilayer wiring substrate is used to reduce the size of a head substrate, in order to ensure insulation of electrodes, it is necessary to increase the thickness of an interlayer insulation layer. In an inkjet-type recording device, if the thickness of an interlayer insulation layer is increased in such a manner, there is a possibility that an excessive amount of heat may be stored in the insulation layer, and the ejection amount of ink may be changed under the influence of heat, resulting in a degradation of image quality and the like. Furthermore, a protection film is disposed over the heating portions, the protection film protecting the heating portions from moisture, etc. However, it is known that if the surface temperature of the heating portions increases excessively, the protection film is degraded due to thermal stress. In order to dissipate the heat of heating portions, a structure including a heat conduction layer is disclosed in Japanese Patent Laid-Open No. 2005-280179.
An inkjet head substrate disclosed in Japanese Patent Laid-Open No. 2005-280179 is shown in FIG. 9. The head substrate shown in FIG. 9 includes a heating region 27a which is used as a heating portion, a second interlayer insulation layer 26 which is composed of a SiO2 film and disposed under the heating region 27a, and a heat conduction layer 35 disposed in the second interlayer insulation layer 26, the heat conduction layer 35 being configured to dissipate heat. The heat conduction layer 35 is composed of a material that has higher thermal conductivity than the second interlayer insulation layer 26. The heat conduction layer 35 is opposed substantially in parallel to the heating portion and is larger, by a predetermined distance a, than the circumference of the heating portion. In such a structure, heat of the heating portion is diffused in a planar direction, and as a result, degradation of the surface of the protection film can be suppressed.
In recent years, there have been demands for higher speed, higher image quality, and higher durability in recording devices. In order to meet such demands, there has been a need for head substrates in which recording elements are arranged at a higher density. The recording elements each include a heating portion and an ejection port, and it is necessary to arrange heating portions at a high density in order to provide recording elements at a high density. When high-speed recording is performed using such a head substrate, even if the heat conduction layer 35 is disposed in the second insulating layer as disclosed in Japanese Patent Laid-Open No. 2005-280179, heat generated in the heating portion is not selectively transmitted downward to a region in which the heat conduction layer 35 is located, and there is a possibility that heat dissipation may not be performed efficiently. If heat dissipation is not performed efficiently, the temperature of the entire substrate may be increased because of thermal interference between the adjacent heating portions arranged at a high density, resulting in a shift of ink ejection timing and other problems, which lead to a degradation in recorded image quality.